The main gap arises from the concept of "unique" visitors. A person using the same browser and computer will, in principle, count as one in Marketo and Google. The gap can be as high as 25% because browsing habits, setting a new cookie for a new browser and/or computer.

In order to get around these potential inaccuracies, you can create a custom report with a non-date dimension that will be the same across sessions for users (e.g., Browser), thereby ensuring Calculation #2 is used. ↑

2 Note that for some dimensions, such as Source or Medium, it is possible that the same unique visitor can be in multiple buckets (e.g., someone visited from organic search and paid search in the same date range). For this reason, when viewing Unique Visitors over such a dimension, the sum of the rows should not add up to the total.